Apoptotic cell death is an evolutionarily conserved process by which organisms remove cells that are superfluous, that have outlived their usefulness, or that are dangerous for the survival of the organism. Deregulation of cell death is associated with a number of human diseases, including cancer and autoimmuity (decreased in cell death), and neurodegenerative diseases (increased cell death). Essentially all of the identified cell death regulatory genes encode proteins. Screens for protein-encoding genes made sense given what we knew at the time - that proteins are the dominant effectors and regulators of cellular functions. However, within the last several years it has become clear that genomes of plants and animals contain on the order of hundreds of small, noncoding RNAs (miRNAs) that regulate gene function through interactions with target mRNAs. A number of these are evolutionarily conserved. The functions of almost all of them are unknown. [unreadable] We have identified multiple Drosophila miRNAs that encode potent cell death inhibitors. At least one of these has a clear human homolog. These observations are important because they define new points and mechanisms of cell death regulation. In addition, they suggest a heretofore hidden resource of cell death regulators whose deregulation may contribute to human disease. For example, death-inhibiting miRNAs, being very small and noncoding, would not have been identified in previous screens for genes that promote oncogenesis by inhibiting cell death. They also would have been missed in experiments designed to identify candidate oncogenes through transcriptional profiling of normal and transformed cells because these experiments were not designed to detect miRNAs. In this proposal we describe genetic and biochemical experiments designed to explore the roles that miRNAs play as cell death regulators. Our efforts will be focused on several goals: to identify evolutionarily conserved cell death-inhibiting miRNAs, to determine the mechanisms by which these miRNAs function - the identities of their mRNA targets - and the contexts in which they are important. We will also search for human homologs of the miRNAs we identify. Where studied, signaling pathways in flies and mammals utilize similar components and regulatory mechanisms. Therefore, it is likely that successful completion of the proposed experiments will increase our understanding of how cell death is regulated in human health and disease. [unreadable] [unreadable] [unreadable]